The discovery of ceramic compositions having superconducting properties is of recent origin. Orginally, superconductivity was observed in mercury at 4K by the Dutch scientist, Heike Onnes. The term, superconductivity, refers to the property wherein a normally resistive conductor abruptly loses all resistance to electrical flow at a specific temperature, called the critical temperature, T.sub.c. At this point, the resistivity of the normal conductor becomes zero.
In more recent times, Ogg (1946) studied superconductivity in quenched metal-ammonia solutions and proposed that superconductivity arose because of mobile electron-pairs.
Until recently, it was believed that superconductivity above 23K, was not possible. This belief was based on the theoretical work of Bardeen, Cooper and Schieffer (BCS theory-1946) which predicted such a limit. Several theoretical proposals were presented in the 1970's, suggesting that the critical temperature for superconductivity could be increased. However, the lack of any discoveries of superconductivity above 23K solidified the belief that indeed this temperature could not be exceeded.
Thus, in November, 1987, when Bednorz and Muller announced the discovery of a new ceramic superconducting compound based on lanthanum, barium, and copper oxides, whose critical temperature for superconductivity was close to 35K., (G. Bednorz and A. Muller, Z. Phys., B64 189 (1986), the declaration was greeted with considerable scepticism. Nevertheless, by the following month, the critical temperature, T.sub.c, for the onset of superconductivity was raised to nearly 80K by C. W. Chu and coworkers (M. K. Wu, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58 908 (1987). This was achieved by changing the composition to yttrium barium copper oxide, approximated by the formula: EQU Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3
This formula, determined experimentally, is not exactly stoichiometric. It is believed that this lack of specific stoichiometry contributes significantly to the onset of superconductivity.
The mechanism of superconductivity in such oxide-based ceramic materials is not at all well understood. Ogg's original contribution suggested that superconductivity arose in quenched metal-ammonia solutions because of mobile electron pairs. The concept accepted at present is similar (the BCS theory), and suggests that if a mobile electron propagates through a lattice structure, it will normally interact with the bound electrons of the lattice because of differences in the electron quantum spin number. However, if two such electrons form a pair which are bound through opposite spin-pairing (Cooper pairs), then no quantum interaction of the bound pairs can occur with the electrons of the lattice (which still have an electron moment).
The so-called 1:2:3 compound, composed of Y-Ba-Cu-O atoms, is prepared by the solid state reaction of the requisite oxides, vis: EQU Y.sub.2 O.sub.3 +2BaO+3CuO=2YBa.sub.2 Cu.sub.3 O.sub.6.5.
It is now established (C. N. Rao et al., Nature, 327 185 (1987) that high T.sub.c superconductivity in the Y-Ba-Cu-O system originates from a compound of stoichiometry: YBa.sub.2 Cu.sub.3 O.sub.7- , where " " is a value less than 1.0. This compound has the structure of the ideal perovskite, TBa.sub.2 Cu.sub.3 O.sub.9. Thus, the superconductor YBa.sub.2 Cu.sub.3 O.sub.7- has about 25% fewer oxygen atoms present in the lattice as compared to the idealized cubic perovskite structure. This massive oxygen deficiency means that instead of the conventional three-dimensional crystalline cubic-stacking array of the perovskite, a unique layered structure results. A loss of even more oxygen atoms in this structure gives rise to the semiconductor YBa.sub.2 Cu.sub.3 O.sub.6. The chain of copper atoms associated with a chain of oxygen atoms is believed to be the key to superconducting behavior. Yet the above description is an idealized one and the actual distinct structural conformation has not yet been delineated. Note that there appear to be extra oxygen atoms in the superconducting unit cell, compared to that of the semiconductor.
To date, most of the high-T.sub.c superconducting ceramic compositions announced to date are based on cuprate compounds having Cu-O.sub.2 layers as part of the structure. Some of these have included:
Bismuth Strontium Calcium Copper Oxide: EQU Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y EQU T.sub.c =114K.
Thallium Calcium (Barium) Copper Oxide: EQU Tl Ba.sub.2 Ca Cu.sub.2 O.sub.7 EQU Tl Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 EQU Tl Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.11 EQU Tl Ba.sub.2 Ca.sub.4 Cu.sub.5 O.sub.13 EQU T.sub.c =120K.
Lead Strontium Lanthanide Copper Oxide EQU Pb.sub.2 Sr.sub.2 (Nd.sub.0.76 Sr.sub.O.24)Cu.sub.3 O.sub.8+x EQU T.sub.c =77K.
In the last compound given, the CuO.sub.2 --sheets are present but there is also a PbO-Cu-OPb sandwich as well, not observed in ceramic superconductors heretofore. The copper ions in this sandwich are monovalent and each is coordinated, above and below, to two oxygen atoms in the PbO layers. The copper atoms in the CuO.sub.2 sheets have an average valence of about 2.25, which is consistent with previously discovered cuprate compounds, given above. However, the presence of Cu.sup.+ atoms lowers the average valence of copper ions in the new structure to below 2.0, which is typical. Indeed, preparation conditions needed to prepare these compounds includes a mildly reducing atmosphere so as not to oxidize Pb.sup.2+ to Pb.sup.4+.
There have also been some compositions announced, based on a copper-free composition, vis: EQU BaO-K.sub.2 O-Bi.sub.2 O.sub.3
This compound is said to become superconducting at about 30K. While copper-oxide superconductors exhibit layered structures that carry current efficiently only along certain planes, this new material is a three-dimensional network of bismuth and oxygen with properties that are much less sensitive to crystallographic direction. It is hoped that compositions will be discovered in this system with temperature properties that rival those of copper-bearing compounds.
The main advantage to superconducting compositions with higher T.sub.c values is that they can operate at liquid nitrogen temperature (78K), thus avoiding the need to use liquid helium. Superconducting ceramic compositions are normally prepared by weighing out specific quantities of selected oxides. The combination is thoroughly mixed by conventional means and then fired at elevated temperatures above about 950.degree. C. The induced solid state reaction causes the formation of the desired ceramic composition and structure. Further annealing in an oxygen atmosphere has been found to improve the superconducting properties of the Y-Ba-Cu-O compound. The produced powder is then processed by convention means to form a bar (by compaction) which is then used as the superconducting medium.
A method to prepare a superconducting film, particularly for use on a silicon substrate as an integrated circuit, has been to deposit thin layers of the appropriate metal oxides in specific order by electron-beam evaporation. Copper is first deposited, then barium, and then yttrium. The sequence is repeated 6-times to obtain an "18-layer" stack of the three ingredients having a total thickness of 0.6-0.7 microns. To complete the process, the specimens are then annealed in oxygen atmosphere for five minutes and then cooled at a rate of about 120.degree. C. per hour.
It is necessary to deposit a buffer layer of inert zirconia on the silicon substrate, before the oxides are deposited, in order to prevent the oxides from reacting with the silicon substrate before the superconducting composition formed. The annealing step was shown to be extremely critical since the oxygen content in the film must be precisely maintained within certain (unknown) limits for the superconductivity state to prevail.
Another approach to preparation of superconducting films has been to employ compounds which are volatile and to cause them to decompose on a hot surface in a partial vacuum. This method is well known for use in the preparation of integrated circuits on a silicon substrate. It is known as vapor phase epitaxy and is capable of producing a superconductive monocrystalline film, using halogen compounds (or others) as the source materials.
Still another method to form a superconducting film is to prepare a superconducting powder of Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.7-x composition, using conventional means. The initial preparation is checked for superconducting properties by measuring a pressed and sintered pellet. Once the material is found to have the desired properties, a powder slurry is made and the slurry is applied with a spin-coater. The layer is dried and then fired in an oxygen atmosphere. Best films are obtained when fired at 940.degree.-1000.degree. C. If sapphire is used as the substrate, the adherence was such that the films could be ground and polished. One could then etch the film with a laser to obtain a desired geometry of superconducting lines, similar to those of a printed circuit.